1. Field of the Invention
This invention lies in the field of submerged burning devices. More particularly, it concerns a submerged burner for heating water, which is utilized immediately for heat transfer to a vaporizable liquid.
Still more particularly, this invention can be used for, or is in the field of, apparatus for vaporizing liquid fuels and the like.
2. Description of the Prior Art
It is well known in industry that volatile fuels, which are stored or retained in liquid phase, must be vaporized prior to gaseous phase burning in burners, which are designed for gaseous fuels. Also, non fuels, such as nitrogen or oxygen, are stored or retained in liquid phase and require vaporization prior to the normal use.
The invention to be described is a vaporizer for such liquids, which may be liquified natural gas, liquified petroleum gases, other hydrocarbon liquids or liquid fuels, as well as oxygen, nitrogen or other non-fuel liquids. The type of liquids to be vaporized are those which have a boiling point which is lower than that of water, so that hot water can be used as the medium of heat transfer from flame products to the liquid to be vaporized.
The art on vaporization shows a number of devices of various forms. Some of the devices rely on thermally-induced convection for heat transfer from water through tubes to a liquid for vaporization. Others rely on turbulent movement of a mixture of hot gases and hot water over the tubular heat transfer surfaces. Such a one is illustrated in U.S. Pat. No. 3,138,150. Still others inject hot combustion gases directly to the liquid, which is vaporized.
In such devices, particularly those in which heat transfer is from water to tubes or pipes, which contain the liquid for vaporization, there are problems of considerable magnitude. The first is the need for maximal rate of heat transfer to the tubes to minimize structural size. The second is the very low temperature of some of the liquids and the danger of freezing water on the tubes to render them inoperative. For example, the temperature of liquified natural gas is less than -258.degree. F., while that of liquid nitrogen is less than -320.degree. F. and rapid movement of water over the tubes is demanded to avoid freezing of the water heat medium. Thus, thermally-induced convection movement of the water is just barely adequate at the expense of extra tubular heat transfer area, as compared to a system which causes much more brisk movement of the water heat medium.
If the heat transfer from the water to the tubes is due to convection, heat transfer by convection will vary as a power of the water flow velocity. In other words, if the water flow velocity can be doubled at constant temperature, say, from 1 foot per second to 2 feet per second, heat transfer to the tube will be increased by 41%, to both reduce the need for heat transfer surface, and better avoid freezing. The water content of the vaporizer is heated, either directly or indirectly, by causing flow of hot combustion gases, either directly into the water or by firing into a structure which is immersed in the water for structure heat transfer to the water. The former is much more efficient and requires the burning of less fuel per unit quantity of vaporized liquid for, improved energy conservation.
Means for causing high velocity water movement within the heat transfer area is a much sought function, and particularly, if the velocity can be accomplished without expenditure of additional energy. The phenomenon of "gas lift" pumping of water is admirably suited here. The combustion gases produced as fuel burns, provides the gas volume for "gas lift" pumping action, as well as the heat which is required for vaporization. This feature is present in this invention, as well in U.S. Pat. No. 3,138,150.
The prior art, and particularly that of U.S. Pat. No. 3,138,150, provides the heat exchange pipes which carry liquid for vaporization immersed in a mixture of gas and water. In this invention, the gas is delivered to the water in a second plenum and the heat of the hot water is retransferred to the tubes of liquid to be vaporized, in a third plenum. In this process, there is not contact of gas with the tubes, and only gas-free water is in contact with the heat exchange tubes. This is done for a number of reasons. First, research shows clearly that the gas temperature, where the gas is very hot, drops to very close to the water temperature within a distance of six inches of the point of gas-to-water injection. Therefore, the gas and water are essentially at the same temperature. Those versed in the art of heat transfer know that, at a selected temperature level, the heat transfer capability of a gas to a metal surface is a small fraction of the heat transfer capability of water to a metal surface. Therefore, the presence of gas in the water contacting a heat transfer surface actually interferes with, and reduces the rate of heat transfer.
It is true that in the case of more rapid movement of the gas, certain turbulence is created within the water. However, greatly increased turbulence is required to compensate for lack of heat transfer capability due to the presence of the gas within the heat transfer area.